1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a touch panel sensing circuit.
2. Description of Related Art
FIG. 1 is a schematic diagram of an n×m touch panel, where n, m are each an integer larger than 1. As shown in FIG. 1, a pulse is inputted into a sensing line X1 110 at one direction (X axis), and charge is coupled to a sensing line Y1 130 at the other direction (Y axis) through mutual capacitance 120 between the sensing lines 110 and 130 in X-axis and Y-axis directions, respectively. Accordingly, a sensing circuit 140 can sense a charge variation on the sensing line Y1 to thereby produce a voltage signal Vo_1. The variation of the voltage signal Vo_1 is used to find the capacitance variation of the mutual capacitance 120 which can further determine whether an object approaches the position (X1, Y1) on the touch panel. Thus a sensing operation is performed on the touch panel.
FIG. 2 is a block diagram of a sensing circuit 140 disclosed in U.S. Pat. No. 6,452,514 granted to Philipp for a “Capacitive sensor and array”. As shown in FIG. 2, the circuit includes a signal generator 210, a first driven electrode 220, a coupling capacitance 230, a second receiving electrode 240, a sampling switch 250, a charge integrator 260, an amplifier 270 and a reset switch 280. The coupling capacitance 230 is provided to induce the electric field change between the sensing lines in X and Y directions. The charge integrator 260 is a capacitor for performing integration.
The signal generator 210 is provided for producing a clock signal. The first driven electrode 220 is connected to the signal generator 210 in order to receive the clock signal. The second receiving electrode 240 is a sensing line in Y-axis direction. The operation of the sensing circuit 140 is essentially implemented by a clock control approach and uses the coupling capacitance 230 to couple the charges to the charge integrator 260, so that the amplifier 270 can use the charge integrator 260 in voltage detection.
However, such a sensing circuit 140 does not consider the parasitic capacitance (Cy or Cx) of sensing lines in X-axis direction or Y-axis direction. The coupling capacitance 230 is typically of the scale of several picofarads, but the parasitic capacitance (Cy or Cx) of a sensing line is up to tens of picofarads. Due to the parasitic capacitance effect (Cy or Cx), the coupling capacitance 230 and the charge integrator 260 have little coupling charge amount in division, resulting in producing a smaller variation to the output voltage Vout. Thus, the amplifier 270 is prone to have a detection problem. In addition, the divided voltage stored in the charge integrator 260 is expressed by a coefficient of
      Cxy          Cs      +      Cxy        ,which is smaller than one, so that the accumulated charge difference is reduced as the number of clock cycles increases. As a result, an additional number of clock cycles is required for obtaining the accumulated charge amount to be detected by the amplifier 270, which further reduces the sensing speed or sensitivity of the sensing circuit of the touch panel. Therefore, there still are problems existed in the conventional skill for sensing a touch on the touch panel, and accordingly it is desirable to provide an improved touch panel sensing circuit to mitigate and/or obviate the aforementioned problems.